PROJECT SUMMARY: During the immune response activated antigen-specific CD8 T cells differentiate into effector cytotoxic T lymphocytes (CTLs) that can recognize and directly kill pathogenic (infected and malignant) cells. Only a few antigen specific T cells that can survive through the primary immune response, forming long-lived memory cells that are critical for secondary encounter with the antigen. Unfortunately, in some pathological conditions such as chronic viral infections and cancer, CTL immune responses are compromised. Specifically, persistent antigen exposure and inflammation prevent the formation of memory T cells and drive effector CTLs into an over-differentiated state where they become so-called ?exhausted? cells with greatly diminished cytotoxic function. Despite extensive research, the mechanisms that regulate these processes are not fully understood. We have identified a novel post-transcriptional mechanism in CD8 T cells that regulates the differentiation of CTLs and is guided by let-7 microRNAs. We found that low expression of let-7 in CD8 T cells enhances differentiation, while high levels suppresses it. We hypothesize that the modulation of let-7 during the differentiation of CD8 T cells is critical for the generation of exhausted and memory T cells, and that let-7 has a unique pattern of expression in each population. We propose a model where persistent antigen stimulation inhibits the expression of let-7 in differentiating CD8 T cells, compromising the generation of functional CTLs and memory T cells. Aim-1 will identify the role of let-7 microRNA expression in the differentiation of effector CTLs into exhausted cells. Aim-2 will determine the role of let-7 in the generation of memory T cells. To address these aims we will analyze the differentiation of CD8 T cells with different levels of let-7 microRNAs into exhausted and memory T cells using a tumor model. Furthermore, using this unique system we will characterize the molecular mechanisms by which let-7 affects the differentiation of exhausted and memory T cells. It is anticipated that these experiments will define let-7 as a new therapeutic target.